Exhaust backpressure
Discussion
GreenV8S said:
Raising the backpressure at low RPM to give you balanced pressure prevents blow-through and hence increases power.
Ok then. Try this.
The exhaust valve opens. High pressure exhaust gas blasts its way down the exhaust header; a pressure gauge plumbed into the header measures 10 psi.
A fraction of a second later and we're into overlap; both valves are now open.
Pressure developed by the supercharger/turbo whatever measures 7 psi as it goes into the cylinder.
10-7= 3psi of pressure(BOOST level with 10 psi backpressure).
We remove the restricted exhaust system and plumb in a mythical zero backpressure exhaust( demon tweeks or halfrauds special
) and now we get the following. Pressure measured in the exhaust reads zero, the same 10 psi boost pressure as before is also present at the intake.
Guess what? 10-0= ? 10 psi boost getting INSIDE the cylinder, thats 7psi more than the previous exhaust.
Now if you still doubt the validity of the statement, do the following practical experiment to convince yourself.
Take a hosepipe, attach it to a pressure gauge on a "tee" and with a means to shut off its flow at the nozzle end.
Then take a 25litre drum, turn the tap full on with the tap at the nozzle fully OPEN and read off the pressure while timing how long it takes to fill the 25 litre drum.
Once complete, chuck the water away and write down the figure you got for the pressure and the time.
Then do it again with the tap closed off (restricted) to say halfway (experiment may be required) then check the pressure and see how long it takes again to fill the 25 litre drum.
I will tell you the result. The first scenario will have a LOW pressure registered and a LOT of flow, resulting in a fast time to fill the drum.
The second scenario will give a HIGH pressure (backpressure) and will take much longer to fill.
This is exactly the situation faced by an engine when operating.
Ie; Only so much time to fill/empty the cylinder ( the 25 litre drum) with only a certain amount of pressure available to get the job done.
Any restriction in the exhaust will hinder the engines capability to exduce its spent gasses, and is seen as a pressure rise in the pipe.
Backpressure is bad bad and bad again and is an unwanted effect on any engine.
Regards deltaf.
I must say i've struggled with the whole 'is back pressure necessary' question too.
Looks like for NA engines we're all agreed that it's not necessary.
For turbo'd engines I can't see why it's necessary, but in any case surely you've got a sodding great impellor in the way so you'll have back pressure anyway whether you like it or not. Downstream of the turbo I can't see any benefit in having a restriction though - actually any pressure here is going to reduce the efficiency of the turbo surely?
As for supercharged engines i'm not sure - looks like we've narrowed the issue down to the time when both valves are open and hence the amount of time the supercharger has a lower pressure on its 'exhaust' side. No idea what difference this might make.
On the subject of tuned exhausts I remember reading years ago that different manifolds (4-2-1 versus 4-1) gave the engine different characteristics - what's that all about then? Is that all about the tuned lengths of the various bits of the manifold or doesn't it matter what length the pipes are, just the design?
Answers, I need answers!
Looks like for NA engines we're all agreed that it's not necessary.
For turbo'd engines I can't see why it's necessary, but in any case surely you've got a sodding great impellor in the way so you'll have back pressure anyway whether you like it or not. Downstream of the turbo I can't see any benefit in having a restriction though - actually any pressure here is going to reduce the efficiency of the turbo surely?
As for supercharged engines i'm not sure - looks like we've narrowed the issue down to the time when both valves are open and hence the amount of time the supercharger has a lower pressure on its 'exhaust' side. No idea what difference this might make.
On the subject of tuned exhausts I remember reading years ago that different manifolds (4-2-1 versus 4-1) gave the engine different characteristics - what's that all about then? Is that all about the tuned lengths of the various bits of the manifold or doesn't it matter what length the pipes are, just the design?
Answers, I need answers!
Any restriction, after the turbine will reduce its efficiency. Fact.
Yes the turbines does cause a restriction, but the gains created by this, far outweigh its negative effects. Restriction can be minimimised with proper sizing, wastegate, and manifold design.
Different style exhaust manifolds work differently due to the pulse tuning/extraction methods. a 4-1 style may work better for say 4-6000rpm, but not be as efficient at the lower rpm side of things, where as the 4-2-1 may work better below 4000rpm, but sacrifice power at the upper rpm's
Depends what your intentions are. Both may have the same overall flow abilities, but there is more going on inside an engine that just that. I wont pretend to understand it all myself.
Yes the turbines does cause a restriction, but the gains created by this, far outweigh its negative effects. Restriction can be minimimised with proper sizing, wastegate, and manifold design.
Different style exhaust manifolds work differently due to the pulse tuning/extraction methods. a 4-1 style may work better for say 4-6000rpm, but not be as efficient at the lower rpm side of things, where as the 4-2-1 may work better below 4000rpm, but sacrifice power at the upper rpm's
Depends what your intentions are. Both may have the same overall flow abilities, but there is more going on inside an engine that just that. I wont pretend to understand it all myself.
I'll have one more go at explaining just for the heck of it. If you still don't get it then I guess we'll just have to agree to disagree, life's too short and all that.
With a turbo, the exhaust turbine produces back pressure similar to the amount of boost in the inlet manifold. This happens more or less regardless of which size turbo you have, what rpm the engine is doing, what boost level you are producing and so on, it is simply a consequence of the relative size and efficiency of the exhast and inlet turbines. If a given turbo produces more back pressure in the exhaust than it produces boost in the inlet manifold then it is worth reducing the back pressure. However, reducing the back pressure down to zero is neither possible (because you have to extract enough power to drive the compressor) nor desireable (because this leads to blow through). To sum up, blow through is not a problem in a turbo application because the turbo itself produces enough back pressure to prevent it. Any additional back pressure caused by restriction in the exhaust is counter productive because it reduces the pressure drop available across the exhaust turbione and hence reduces the power available to drive the compressor. Because blow-through is not a problem with turbo applications, they can use fairly conventional cams. How often do you find turbo engine designers agonising over cam design? No more than for NA engines, because the cam designs can be very similar.
Superchargers are a different matter altogether. There is nothing to produce back pressure other than any pressure loss caused by restriction in the exhaust - usually very low at low rpm. During overlap, compressed air in the inlet manifold can blow through to the exhaust. Any charge that ends up in the exhaust is lost and wasted, it means the energy spent compressing it has been wasted, and also means that there is less charge remaining in the cylinder and hence less torque from the engine. You don't need to think in terms of barrels and hose pipes and taps, just consider that your s/c puts a fixed amount of charge into the inlet manifold each revolution, and any that manages to get through the engine without being burned in the cylinder represents lost cylinder filling and lost torque. Blow through happens at low rpm where inertial effects are not enough to be significant. At higher RPM as the inertial effects become more significant the blow-through problem goes away, in fact the pressure differential between inlet and exhaust promotes scavenging at higher rpm than you would get it with a n/a or turbo engine; supercharged engies do not tend to suffer from blow through at high RPM. To counter the blow-through problem, s/c engine builders tend to run cams with very low overlap. This produces problems of its own, because these short period cams run very high ramp loads and it is very difficult to get the cams to last. It also means that the volumetric efficiency at high rpm, where blow through is not a problem, has been compromised. With low overlap, the engine doesn't breath as well at high rpm. With a positive displacement supercharger it doesn't care how much boost it takes to get the charge in, it will produce as much pressure as necessary in the inlet manifold to get that charge into the cylinder. This leads to thermal runaway where the natural volumentric efficiency of the engine drops off, so the supercharger has to put more energy in to pressurize the charge, this increases the charge temperature which increases the inlet manifold pressure which means the s/c has to put even more energy in, the charge temperature gets higher and higher until eventually something goes pop (or the engine designer is sensible, backs the ignition off, the torque goes through the floor but the engine survives). Talk to any engine builder who has had a serious go at designing a s/c engine and they will tell you what a nightmare it is to get a cam to work properly. All caused, ultimately, because of a pressure imbalance between the inlet and exhaust. More back pressure at low RPM corrects this imbalance, without requiring daft stumpy cams that screw up the top end.
Hope this makes sense, it not :shrug:
wouldn't it be a boring world if we all agreed about everything all the time!
With a turbo, the exhaust turbine produces back pressure similar to the amount of boost in the inlet manifold. This happens more or less regardless of which size turbo you have, what rpm the engine is doing, what boost level you are producing and so on, it is simply a consequence of the relative size and efficiency of the exhast and inlet turbines. If a given turbo produces more back pressure in the exhaust than it produces boost in the inlet manifold then it is worth reducing the back pressure. However, reducing the back pressure down to zero is neither possible (because you have to extract enough power to drive the compressor) nor desireable (because this leads to blow through). To sum up, blow through is not a problem in a turbo application because the turbo itself produces enough back pressure to prevent it. Any additional back pressure caused by restriction in the exhaust is counter productive because it reduces the pressure drop available across the exhaust turbione and hence reduces the power available to drive the compressor. Because blow-through is not a problem with turbo applications, they can use fairly conventional cams. How often do you find turbo engine designers agonising over cam design? No more than for NA engines, because the cam designs can be very similar.
Superchargers are a different matter altogether. There is nothing to produce back pressure other than any pressure loss caused by restriction in the exhaust - usually very low at low rpm. During overlap, compressed air in the inlet manifold can blow through to the exhaust. Any charge that ends up in the exhaust is lost and wasted, it means the energy spent compressing it has been wasted, and also means that there is less charge remaining in the cylinder and hence less torque from the engine. You don't need to think in terms of barrels and hose pipes and taps, just consider that your s/c puts a fixed amount of charge into the inlet manifold each revolution, and any that manages to get through the engine without being burned in the cylinder represents lost cylinder filling and lost torque. Blow through happens at low rpm where inertial effects are not enough to be significant. At higher RPM as the inertial effects become more significant the blow-through problem goes away, in fact the pressure differential between inlet and exhaust promotes scavenging at higher rpm than you would get it with a n/a or turbo engine; supercharged engies do not tend to suffer from blow through at high RPM. To counter the blow-through problem, s/c engine builders tend to run cams with very low overlap. This produces problems of its own, because these short period cams run very high ramp loads and it is very difficult to get the cams to last. It also means that the volumetric efficiency at high rpm, where blow through is not a problem, has been compromised. With low overlap, the engine doesn't breath as well at high rpm. With a positive displacement supercharger it doesn't care how much boost it takes to get the charge in, it will produce as much pressure as necessary in the inlet manifold to get that charge into the cylinder. This leads to thermal runaway where the natural volumentric efficiency of the engine drops off, so the supercharger has to put more energy in to pressurize the charge, this increases the charge temperature which increases the inlet manifold pressure which means the s/c has to put even more energy in, the charge temperature gets higher and higher until eventually something goes pop (or the engine designer is sensible, backs the ignition off, the torque goes through the floor but the engine survives). Talk to any engine builder who has had a serious go at designing a s/c engine and they will tell you what a nightmare it is to get a cam to work properly. All caused, ultimately, because of a pressure imbalance between the inlet and exhaust. More back pressure at low RPM corrects this imbalance, without requiring daft stumpy cams that screw up the top end.
Hope this makes sense, it not :shrug:
wouldn't it be a boring world if we all agreed about everything all the time!More than happy with what you're saying there Peter, I have no doubts you've gone into this with your eyes wide open and your ears pinned back.
Re the cam, its one of the things that has always confused, me with the RV8. The 218 is in effect milder than the 214 when you look at the lift and the overlap. Although it does have the higher number, in 'Kentcams' speak. In the actual installation it out performs the 214 though
.
btw Have you changed your cam at all recently, given the route you are taking, if so what for.
Can I assume you may have trick exhaust as well
Harry
Re the cam, its one of the things that has always confused, me with the RV8. The 218 is in effect milder than the 214 when you look at the lift and the overlap. Although it does have the higher number, in 'Kentcams' speak. In the actual installation it out performs the 214 though
. btw Have you changed your cam at all recently, given the route you are taking, if so what for.
Can I assume you may have trick exhaust as well
Harry
You have a very strange understanding of how engines work....
Virtually all engines that create good power figures, have exhausts designed for as little restriction as possible, within certain design restricitons. eg cats, noise.
Where these rules are not required, most have open exhausts, for zero restriction.
Thats fact.
Virtually all engines that create good power figures, have exhausts designed for as little restriction as possible, within certain design restricitons. eg cats, noise.
Where these rules are not required, most have open exhausts, for zero restriction.
Thats fact.
HarryW said:
Re the cam, its one of the things that has always confused, me with the RV8. The 218 is in effect milder than the 214 when you look at the lift and the overlap. Although it does have the higher number, in 'Kentcams' speak. In the actual installation it out performs the 214 though
Somebody, I think it was Tim Lamont but don't quote me, advised me to think of the 214H as a 234 in disguise in terms of its tuning characteristics. I had one in the 4 litre and it was a screamer, but the low end torque really suffered. The 218 may not produce the peak numbers but what good is it putting out power at 6500 rpm in a road car? The 218 has a much broader torque with a great 'Martini' effect, I really like it and I have no immediate plans to change.
>> Edited by GreenV8S on Wednesday 15th September 23:34
GreenV8S said:
... The 218 may not produce the peak numbers but what good is it putting out power at 6500 rpm in a road car? The 218 has a much broader torque with a great 'Martini' effect, I really like it and I have no immediate plans to change.
If I knew what the 'Martini' effect was I might agree
. I would have thought with its smaller overlap, something like 57 degrees as opposed to 80 odd degrees for the hotter cams it would lend itself well to forced induction too
. Harry
I can think of at least two workarounds for blow-through at low rpm on a supercharged engine. One is, as has been said, to arrange for a positive pressure pulse to arrive at the exhaust valve during the overlap period at the troublesome rpm by suitable design of the exhaust system. The other possibility, not quite so nice, is to obtain the required exhaust duration by opening the exhaust valve earlier. Most of the power is developed before 90 deg ATDC, so this is less lossy than it may sound.
Peter,
Excellent stuff - thanks for taking the time to explain some of that stuff. In the interests of conversaion and certainly not to 'have a go' so to speak I have some comments tho.
You said:
I guess the point about losing efficiency in a supercharged engine when the blower is effectively blowing straight through the engine into the exhaust has got to make sense. But what's the effect of this? A reduction of pressure on the inlet side? How much? I assume this means less volume making it into the cylinder, but again - is this significant? And is it outweighed by the benefits of having a non-overlap cam with no blow through.
I'm back to the same question again: what need is there for valve overlap in a turbo/sc engine?
If there's no valve overlap, then why have any back pressure?
It's all greek to me ...
Excellent stuff - thanks for taking the time to explain some of that stuff. In the interests of conversaion and certainly not to 'have a go' so to speak I have some comments tho.
You said:
GreenV8S said:Surely it's the other way round: turbo applications have no need for valve overlap and so blow-through can't happen. I would have thought overlap is a way of filling the cylinder with as much mixture as possible on the inlet stroke? In a turbo application i'd expect that to happen anyway due to the pressurised inlet? Is overlap overkill for a turbo or s/c engine?
Because blow-through is not a problem with turbo applications, they can use fairly conventional cams.
I guess the point about losing efficiency in a supercharged engine when the blower is effectively blowing straight through the engine into the exhaust has got to make sense. But what's the effect of this? A reduction of pressure on the inlet side? How much? I assume this means less volume making it into the cylinder, but again - is this significant? And is it outweighed by the benefits of having a non-overlap cam with no blow through.
I'm back to the same question again: what need is there for valve overlap in a turbo/sc engine?
If there's no valve overlap, then why have any back pressure?
It's all greek to me ...
Losses on overlap are minmal, unless you are using real racy cams.. And on any engine, such cams have problems, until airspeed sorts them out.
Those same SC overlap losses would also present a totally clean incoming air/fuel charge, with no exhaust contamination. So while not being so fuel efficient, the mixture for burning, is excellent.
Again, the problems you mention, are mostly all related to cam design, trying to fix them by blocking the exhaust is a very strange move indeed.
Your understanding of turbo engines is also strange. You think that boost pressure = inlet manifold pressure. If this was the case, then you would have a very efficient turbo installation, which could ahve the potential for making huge power, albeit it would probably be quite laggy. In most cases you will find though, EGBP can in fact be as much as double IMP, hence low overlap cams are the norm, as reversion/inlet charge contamination is very likely. Some Porsche engines of old, actually had zero overlap.
EGBP in a turbo motor is not desirable, but it is something that is unaviodable due to what a turbo is. Trying to minimise it, without reducing the turbines effiency makes power. If it takes 30psi in the exhaust manifold, pre turbo, to create 15psi of boost in the inlet, then that isnt really ideal.
If however, with good manifold design, and a suitably sized turbine, this could be reduced to say 20psi EGBP, still to create 15psi in the inlet, then suddenly you have made it much more efficient, taking less energy, making less restriction, which in turn will create more power.
Just changing to a larger turbine housing on say a 300bhp engine, could yield as much as 30bhp in some cases, still using teh same inlet boost pressure. Lag may be more apparent, but top end gains, are worth it.
Similarly, changing from a log style manifold, less restrictive tubular desing can also yield massive benefits. The only thing that has changed, is that there will be less EGBP, by making the breathing easier.
Try running, breathing in through your mouth, and breathing out through a straw.
Is restriction good ?
Those same SC overlap losses would also present a totally clean incoming air/fuel charge, with no exhaust contamination. So while not being so fuel efficient, the mixture for burning, is excellent.
Again, the problems you mention, are mostly all related to cam design, trying to fix them by blocking the exhaust is a very strange move indeed.
Your understanding of turbo engines is also strange. You think that boost pressure = inlet manifold pressure. If this was the case, then you would have a very efficient turbo installation, which could ahve the potential for making huge power, albeit it would probably be quite laggy. In most cases you will find though, EGBP can in fact be as much as double IMP, hence low overlap cams are the norm, as reversion/inlet charge contamination is very likely. Some Porsche engines of old, actually had zero overlap.
EGBP in a turbo motor is not desirable, but it is something that is unaviodable due to what a turbo is. Trying to minimise it, without reducing the turbines effiency makes power. If it takes 30psi in the exhaust manifold, pre turbo, to create 15psi of boost in the inlet, then that isnt really ideal.
If however, with good manifold design, and a suitably sized turbine, this could be reduced to say 20psi EGBP, still to create 15psi in the inlet, then suddenly you have made it much more efficient, taking less energy, making less restriction, which in turn will create more power.
Just changing to a larger turbine housing on say a 300bhp engine, could yield as much as 30bhp in some cases, still using teh same inlet boost pressure. Lag may be more apparent, but top end gains, are worth it.
Similarly, changing from a log style manifold, less restrictive tubular desing can also yield massive benefits. The only thing that has changed, is that there will be less EGBP, by making the breathing easier.
Try running, breathing in through your mouth, and breathing out through a straw.
Is restriction good ?
Seems to me GreenV8s needs variable valve timing and a zero backpressure exhaust for his supercharger application: minimise overlap at low rpm to make sure there is no charge loss, and widen the overlap at high rpm to get the torque. You have got that in your plans, haven't you Peter.
stevieturbo said:
Losses on overlap are minmal,
They are not minimal in the supercharged case. As well as losing the boost blow-through can damage the exhaust, lambda sensors, cats etc.
To avoid pages of quotes I will try to summarize where I think we have go to.
The question is whether back pressure is ever beneficial. I suggested that back pressure helps prevent a problem that supercharged engines suffer at low rpm. Somebody commented that this problem didn't actually occur in practice. I explained that it is a problem that doesn't occur with turbo installations because the turbo produces enough back pressure to prevent it. Somebody pointed out that actually turbos produce more back pressure than I said, and they don't suffer from blow-through. Harah, we agree. Somebody suggested that the supercharger problems could be addressed without back pressure by designing the cam right. I have pointed out that this produces a cam that isn't very durable and also cripples top end performance.
I suspect one reason some of you disagree so strongly with what I am saying is that I am talking about back pressure and you are talking about restrictive exhausts. I am not trying to suggest that using a drinking straw as an exhaust is a sensible way to produce back pressure. Quite the opposite: since the back pressure from a restrictive exhaust increases with revs, an exhaust that was restrictive enough to produce the desired amount of back pressure at low rpm (when the back pressure is needed) would produce far far too much at high rpm (where no back pressure is needed). It needs a cleverer solution, fortunately there are a couple to choose from.
Any form of backpressure, measured in the exh manifold, just after the valve, has to increase with rpm's, unless you have some form of variable exhaust opening..
I hear what you are saying re-blow thru with the lambda sensor, but, again, thats cam design.
overlap is overlap. A SC engine should ahve positive pressure available throughout most of teh rpm range, so at any rpm, this blow thru scenario could happen, probably more so at high rpm, when things become more efficient, and air speeds are higher, and boost pressures are higher.
Therefore, cam design should take this into consideration.
I would hardly say that designing a cam to minimise blow thru would cripple an engines performance.
IMO, racy cams with big overlaps, crippler performance. They make make for cammy revvy engines that create power at top end, but who wants that thtese days.
A forced indutcion engine, with a mild cam, will have excellent missions, excellent performance, good fuel economy, and impeccable driveability, from idle, to redline.
Why would anyone want big cams with huge amounts of overlap in the first place ???
I hear what you are saying re-blow thru with the lambda sensor, but, again, thats cam design.
overlap is overlap. A SC engine should ahve positive pressure available throughout most of teh rpm range, so at any rpm, this blow thru scenario could happen, probably more so at high rpm, when things become more efficient, and air speeds are higher, and boost pressures are higher.
Therefore, cam design should take this into consideration.
I would hardly say that designing a cam to minimise blow thru would cripple an engines performance.
IMO, racy cams with big overlaps, crippler performance. They make make for cammy revvy engines that create power at top end, but who wants that thtese days.
A forced indutcion engine, with a mild cam, will have excellent missions, excellent performance, good fuel economy, and impeccable driveability, from idle, to redline.
Why would anyone want big cams with huge amounts of overlap in the first place ???
I cant see how youll get MORE charge into a cylinder if its already half pressurised by backpressure seeping back in thru an open exhaust valve.
Like Stevieturbo suggested, yours is likely a cam problem, not a balance between intake and exhaust pressures.
If you were to shift the position of pressures measured from say the exhaust and then apply it to the cylinder, and then attempt to pump pressure into it, youll end up with a no flow/back flow situation.
Backpressure Isnt needed mate, it really isnt.
I think theres a little confusion here also.
GreenV8S is basically saying that blow through is causing a loss of power on his system,as the pressure is flowing thu on overlap, so its better for him to have some backpressure to act as a barrier for the intake charge to ramp up against; Big problem with such a theory/situation; The same backpressure will then rob you of power when the exhaust valve opens again. Basically youre pumping AGAINST pressure....so much easier to have zero and expel that burnt charge so much faster.
Like Stevieturbo suggested, yours is likely a cam problem, not a balance between intake and exhaust pressures.
If you were to shift the position of pressures measured from say the exhaust and then apply it to the cylinder, and then attempt to pump pressure into it, youll end up with a no flow/back flow situation.
Backpressure Isnt needed mate, it really isnt.
I think theres a little confusion here also.
GreenV8S is basically saying that blow through is causing a loss of power on his system,as the pressure is flowing thu on overlap, so its better for him to have some backpressure to act as a barrier for the intake charge to ramp up against; Big problem with such a theory/situation; The same backpressure will then rob you of power when the exhaust valve opens again. Basically youre pumping AGAINST pressure....so much easier to have zero and expel that burnt charge so much faster.
Most learned participants, could I drag you back once more to my R1.
Its an NA engine with NO cat, yet the exup valve was found to give a considerable boost to the engine.
I contend that it represents an obvious variable restriction to the exhaust, and a simple butterfly would not serve as an exhaust length tuner.
exhaust-ive discussion
http://forums.turbobricks.com/archive/index.php/t-8394.html
>> Edited by Julian64 on Thursday 16th September 19:54
>> Edited by Julian64 on Thursday 16th September 19:54
Its an NA engine with NO cat, yet the exup valve was found to give a considerable boost to the engine.
I contend that it represents an obvious variable restriction to the exhaust, and a simple butterfly would not serve as an exhaust length tuner.
exhaust-ive discussion
http://forums.turbobricks.com/archive/index.php/t-8394.html
>> Edited by Julian64 on Thursday 16th September 19:54
>> Edited by Julian64 on Thursday 16th September 19:54
Julian, a quote from that website:
Bishop said:Perhaps not a restriction then?
Yamaha have been using the "EXUP" valve in the exhaust sytem of their bikes for years, but even in the motorcycle scene people missunderstand it's purpose, with many people claiming it created backpressure in the system to improve low to midrage torque.
The reality is that Yamaha used it to keep exhaust gas velosity high at low to mid rpm, meaning they had 250-1000cc bikes in the 80's that ran rings around the compitition, at low to mid rpm.
deltaf said:
GreenV8S is basically saying that blow through is causing a loss of power on his system,as the pressure is flowing thu on overlap, so its better for him to have some backpressure to act as a barrier for the intake charge to ramp up against; Big problem with such a theory/situation; The same backpressure will then rob you of power when the exhaust valve opens again. Basically youre pumping AGAINST pressure....so much easier to have zero and expel that burnt charge so much faster.
That's what I'm saying, but I think you've over estimated the power loss on the exhaust stroke caused by the back pressure. Look at it from the engine's point of view. All it sees is higher air pressure and density in the inlet and exhaust manifolds. It is exactly the same effect as running your engine at the bottom of a deep mine shaft, except you have achieved it with a supercharger. Yes it will take slightly more power to expell the exhaust, but that's nothing compared to the benefit of having full boost instead of none.
It is true that the problem can be avoided by using a supercharger cam grind with little/no overlap, but that approach brings its own problems. Supercharged engines thrive on overlap at high RPM just like n/a engines do. In some ways it's even more important with a s/c engine because an n/a engine just loses torque when the volumetric efficiency drops off; a positive displacement supercharged engine develops more and more back pressure in the inlet manifold which can lead to thermal runaway and major engine damage. This is one of the reasons why superchargers got themselves a bit of a reputation for blowing up engines. Getting a cam with little/no overlap to breath well at higher rpm is extremely challenging, and leads to far higher cam loads than would normally be tolerated.
diverting away a bit..... but how critical is a the exhaust manifold design/lengths on a turbocharged engine.
I know with an NA you need to tune the manifold.... but do you on a turbo one.
Reason I ask (other than I need to make mine) is whilst pondering the design for mine I was wondering if there is any loss if they were different lengths as they all hit the turbo anyhow?
I know with an NA you need to tune the manifold.... but do you on a turbo one.
Reason I ask (other than I need to make mine) is whilst pondering the design for mine I was wondering if there is any loss if they were different lengths as they all hit the turbo anyhow?
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff